The BA Air Core and BA Insulated Air Core are two quite different pads!
Check the State of the Market report series on sleeping pads that BPL issued last August:
Note especially that the tested R-value varies widely with the amount of inflation. If you inflate your pad fully (assuming you can stand to sleep on that hard a surface), you'll get the highest R value. If you like your pad really squishy (as I do), the tested R-value will be quite a bit less. Probably the average R-value shown in the ratings is as close as you'll get to real-life use.
Clearly, individuals differ as to how much insulation they need underneath. However, I have read that the EN13537 tests of sleeping bags specify a mat with R-value of 5 for a 20*F/-7*C sleeping bag. I haven't been able to confirm this, but it's pretty much what I've found from experience.
From Part 1 of the above cited SOTM article: "If you must have an answer for a winter mat, a minimum R-value of 5 would be a good starting point. Below that and you may have some problems, depending on how you sleep and other conditions as outlined above. A mat with an R-value above 6 should be fairly reliable, even comfortable. Note that what a mat offers does seriously depend on how thick it is in the field, and this is a major topic for Part 2."
Then, of course, there's the difference between frozen ground and snow (the latter is usually warmer once your body heat goes through the pad and starts to melt it).
Also check Mark Verber's encyclopedic website (page down to the sleeping pad section):